The Good Guy Concept
The Doctor as a character no matter what moral grey areas he may go into at some point should always be for the purpose of the greater good, Doctor's have often flip flopped between one moral stand point to the other on various topics but all (or at least the one's considered the real Doctor) have always had the best intentions at heart, even if they weren't always achieved through the best of means, regardless to this any Doctor has to have an element of caring and genuine goodness about them, as it's something every Doctor needs to keep him grounded.